Generally, in an exposure process on manufacturing a semiconductor integrated circuit, it is necessary to write a circuit pattern on a wafer applied with a resist by the use of a mask (it may also be called a reticle) with the circuit pattern written thereon (which is called pattern exposure), and a system therefor is called an exposure system or an exposure apparatus. However, there is also an exposure system for directly writing a circuit pattern on a wafer without using a mask (this system is called a maskless exposure system).
On the other hand, in order to fabricate a mask, it is necessary to deposit, on the surface of a quartz plate or the like which will serve as a substrate of the mask, a chromium film or the like for shielding that is patterned to allow exposure light to pass through a pattern corresponding to an intended circuit pattern or an object circuit pattern. This chromium film or the like is patterned by pattern exposure and a system therefor is called a mask writing system. The technique of the mask writing system is generally electron-beam writing using an electron beam and a system therefor is called an electron-beam writing system (hereinafter referred to as an EB writing system).
In addition to the EB writing system, there is also an alternate mask writing system (sometimes called a laser-beam writing system), which has been manufactured and sold as a product and which is based on a technique that performs pattern writing (i.e. pattern exposure with respect to a mask substrate coated with a resist) by the use of laser light in the ultraviolet region (hereinafter abbreviated as ultraviolet laser light).
As the writing system of this type, there has conventionally been proposed a laser-beam writing system that uses a reflector display element (a mirror device called a digital micromirror or the like) having a large number of micromirrors arranged two-dimensionally and performs pattern writing on a mask substrate by irradiating ultraviolet laser light onto the reflector display element and by controlling reflected light into a pattern. This laser-beam writing system can expose a partial pattern of a circuit pattern at one time and therefore has a high processing speed, as known in the art. Description is given about this, for example, in Proceedings of SPIE, Vol. 4186, PP. 16-21 or U.S. Pat. No. 6,428,940.
According to the above-mentioned documents, the conventional laser-beam writing system uses the mirror device composed of about one million (about 500×about 2000) micromirrors each of which has a size of about 16 microns. This is projected onto the mask substrate at a reduced size of 1/160 through a reduction-projection optical system. As a result, a pattern corresponding to each micromirror becomes a square with a side of 0.1 microns, i.e. 100 nm. On the other hand, when writing a mask, the minimum size in design is generally small like 1 to 4 nm, which is called a minimum grid. Therefore, in order to realize a pattern shape that is far smaller than the mirror-projected pattern with the side of 100 nm, it is performed to change an amount of light irradiated onto a pattern to be projected. For example, according to the foregoing document, the amount of light is changed in 64 levels (using intermediate amounts of light), thereby adapting to a minimum grid of 1.56 nm being 1/64 of 100 nm.
In the conventional technique of adapting to the minimum grid having the size smaller than the reduction-projected pattern of each micromirror by the use of the intermediate amounts of light as described above, a deflection angle of each micromirror in the mirror device is controlled, thereby changing the intensity of laser light to be projected. In this connection, if exposure is performed by moving the micromirror to be projected (i.e. scanning the mask substrate) per minimum grid of 1.56 nm, the scan speed is reduced to 1/64 while the number of scan times increases 64 times, and therefore, a writing time is extremely prolonged like 64×64 times. That is, it is essential to use the intermediate amounts of light in order to shorten the writing time in the laser-beam writing system.